Absorber layer for a thin film photovoltaic device with a double-graded band gap

ABSTRACT

A gallium-containing alloy is formed on the light-receiving surface of a CIGS absorber layer, and, in conjunction with a subsequent selenization or anneal process, is converted to a gallium-rich region at the light-receiving surface of the CIGS absorber layer. A second gallium-rich region is formed at the back contact surface of the CIGS absorber layer during selenization, so that the CIGS absorber layer has a double-graded gallium concentration that increases toward the light-receiving surface and toward the back contact surface of the CIGS absorber layer. The double-graded gallium concentration advantageously produces a double-graded bandgap profile for the CIGS absorber layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a Continuation Application of U.S. patent application Ser. No.13/331,793, filed on Dec. 20, 2011, which is herein incorporated byreference for all purposes.

BACKGROUND

1. Field of the Invention

This invention relates to thin film photovoltaic devices, and moreparticularly, to an absorber layer for a thin film photovoltaic devicethat has a double-graded band gap, and methods of forming the same.

2. Description of the Related Art

Solar cells are photovoltaic devices that convert light into electricalenergy, and have been developed as clean, renewable energy sources tomeet growing energy needs worldwide. The relatively high manufacturingcosts associated with conventional crystalline silicon solar cells,which use thick substrates of high-quality material, is driving thedevelopment of large area thin film photovoltaic (TFPV) devices. TFPVdevices may be formed from thin (<10 micron) films of amorphous,nanocrystalline, micro-crystalline, or mono-crystalline materials, andwhen fabricated on low-cost substrates, provide an economicalalternative to conventional crystalline silicon solar cells.

TFPV devices that employ copper-indium-gallium-selenide (CIGS) absorberlayers are of special interest, since CIGS absorbers have demonstratedhigh lab-cell efficiency (>20%) and large-area module efficiency (>15%).This is because CIGS films have a high absorption coefficient (i.e.,approximately 10⁵/cm), bandgaps in the range of 1.0 eV (forcopper-indium-selenide) to 1.65 eV (for copper-gallium-selenide), andare efficient absorbers across the entire visible spectrum. Furthermore,CIGS absorbers generally consist of earth-abundant materials, makingCIGS-based TFPV devices scalable for high-volume manufacturing.

“Double grading” the bandgap of the CIGS absorber is a method known inthe art to increase the efficiency of CIGS solar cells. In a CIGSabsorber layer that has a double-graded bandgap profile, the bandgap ofthe CIGS layer increases toward the front surface and toward the backsurface of the CIGS layer, with a bandgap minimum located in a centerregion of the CIGS layer. The increasing bandgap profile at the frontsurface of the CIGS layer, i.e., the surface that receives incidentlight, discourages the generation of charge carriers near this surface,thereby reducing unwanted charge carrier recombination before the chargecarriers can reach the appropriate electrode. The increasing bandgapprofile at the back surface of the CIGS layer creates a back surfacefield, which reduces recombination at the back surface and enhancescarrier collection.

Co-evaporation is one technique known in the art for producing adouble-graded bandgap in a CIGS absorber layer. The co-evaporationprocess can produce a gallium (Ga) rich region at the front and backsurfaces of a CIGS absorber layer and a gallium-poor region in thecenter of the CIGS absorber layer. However, co-evaporation is arelatively complex process that is not as economical or as easilyimplemented as other deposition processes known in the art. In a 2-stepprocess, Cu—In—Ga metal precursors are deposited first, followed by asecond selenization process to form a CIGS absorber layer. The 2-stepprocess is generally more suited to large-scale low-cost manufacturingcompared to the co-evaporation process. However, because gallium hasslower reaction kinetics with selenium (Se) than with indium (In),gallium tends to accumulate towards the back surface of the CIGS layerduring the selenization process, thereby creating a single grading inthe bandgap profile, i.e., the bandgap of the CIGS layer increases fromthe front surface to the back surface. Double grading of the bandgapprofile is then typically achieved by the incorporation of sulfur (S) atthe front surface of the CIGS layer. However, sulfur incorporation addsconsiderable complexity to the deposition process and produces a TFPVabsorber material (copper-indium-gallium-sulfur) of lower qualitycompared to CIGS.

In light of the above, there is a need in the art for an economicalmethod of creating a CIGS absorber layer having a double-graded band gapthat does not use sulfur incorporation.

SUMMARY

Embodiments of the invention set forth a method of forming acopper-indium-gallium-selenide (CIGS) absorber layer in a thin filmphotovoltaic device with a double-graded band gap and a double-gradedgallium concentration. In general, a gallium-containing alloy is formedon the light-receiving surface of a CIGS absorber layer, and, inconjunction with a subsequent selenization or anneal process, isconverted to a gallium-rich region at the front surface of the absorberlayer.

According to one embodiment of the present invention, a method offorming an absorber layer for a thin-film solar cell includes the stepsof depositing a copper-indium-gallium layer on a substrate, forming analloy on a first surface of the copper-indium-gallium layer thatincludes gallium (Ga) and selenium (Se), and performing a selenizationprocess on the substrate that converts the gallium-containing alloy to agallium-rich region at the first surface of the absorber, whereinforming the gallium-containing alloy on the first surface of thecopper-indium-gallium layer comprises maintaining the substrate at atemperature lower than a temperature at which the selenium-containinggas reacts with the copper-indium-gallium layer.

According to another embodiment of the present invention, a method offorming an absorber layer for a thin-film solar cell includes the stepsof depositing a copper-indium-gallium layer on a substrate, exposing thecopper-indium-gallium layer to trimethyl gallium (TMGa) and a selenium(Se) containing gas to form a Ga_(x)Se_(y)-containing layer on a firstsurface of the copper-indium-gallium layer, and forming a gallium-richregion at the first surface of the copper-indium-gallium layer byannealing the Ga_(x)Se_(y)-containing layer and converting the depositedGa_(x)Se_(y)-containing layer to the gallium-rich region, whereinexposing the copper-indium-gallium layer to TMGa and aselenium-containing gas comprises maintaining the substrate at atemperature lower than a temperature at which the selenium-containinggas reacts with the copper-indium-gallium layer.

According to yet another embodiment of the present invention, a methodof forming an absorber layer for a thin-film solar cell includes thesteps of depositing a copper-indium-gallium layer on a substrate, thecopper-indium-gallium layer having a gallium-rich region, depositing agallium layer on a first surface of the copper-indium-gallium layer viathermal pyrolysis of TMGa, converting the gallium layer to aGa_(x)Se_(y)-containing layer via a selenization process, and thermallyconverting the Ga_(x)Se_(y)-containing layer to a gallium-rich region atthe first surface of the copper-indium-gallium layer.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above recited features of embodiments ofthe invention can be understood in detail, a more particular descriptionof embodiments of the invention, briefly summarized above, may be had byreference to the appended drawings. It is to be noted, however, that theappended drawings illustrate only typical embodiments of this inventionand are therefore not to be considered limiting of its scope, for theinvention may admit to other equally effective embodiments.

FIG. 1 is a schematic cross-sectional view of a thin film photovoltaicdevice with a copper-indium-gallium-selenide (CIGS) absorber layer,configured according to embodiments of the invention.

FIG. 2 sets forth a flowchart of method steps in a process sequence forforming a CIGS absorber layer, according to embodiments of theinvention.

FIGS. 3A-3C sequentially illustrate cross-sectional views of a TFPVdevice during the execution of the process sequence illustrated in FIG.2, according to embodiments of the invention.

FIG. 4 sets forth a flowchart of method steps in a process sequence forforming a CIGS absorber layer, according to embodiments of theinvention.

FIGS. 5A-5D sequentially illustrate cross-sectional views of a TFPVdevice during the execution of the process sequence illustrated in FIG.4, according to embodiments of the invention.

FIG. 6 sets forth a flowchart of method steps in a process sequence 600for forming a CIGS absorber layer, according to embodiments of theinvention.

FIGS. 7A-7D sequentially illustrate cross-sectional views of a TFPVdevice during the execution of the process sequence illustrated in FIG.6, according to embodiments of the invention.

For clarity, identical reference numbers have been used, whereapplicable, to designate identical elements that are common betweenfigures. It is contemplated that features of one embodiment may beincorporated in other embodiments without further recitation.

DETAILED DESCRIPTION

FIG. 1 is a schematic cross-sectional view of a thin film photovoltaic(TFPV) device 100 with a copper-indium-gallium-selenide (CIGS) absorberlayer 120, configured according to embodiments of the invention. TFPVdevice 100 includes a back contact layer 110, CIGS absorber layer 120, abuffer layer 130, and a transparent conductive oxide (TCO) stack 140,arranged as shown on a substrate 105 to form a TFPV device stack. Light190 is incident on a front surface 143 of TFPV device 100, passesthrough TCO stack 140 and buffer layer 130, and is absorbed by CIGSabsorber layer 120 and converted to electrical energy.

Substrate 105 may be a rigid or flexible substrate. Examples of rigidsubstrates suitable for use as substrate 105 include float glass,low-iron glass, borosilicate glass, specialty glass for high temperatureprocessing, stainless steel, carbon steel, aluminum, copper, titanium,molybdenum, plastics, etc. Examples of flexible substrates suitable foruse as substrate 105 include polyimide, flexible glass, cladded metalfoils, etc.

Back contact layer 110 serves as the primary current conductor layer ofTFPV device 100 and is also configured to reflect most unabsorbed lightback into CIGS absorber layer 120. In one embodiment, back contact layer110 comprises a molybdenum (Mo) layer that has a thickness that isbetween about 0.3 microns and about 1.0 microns. In addition to highreflectivity, it is desirable for back contact layer 110 to haverelatively high electrical conductivity, good ohmic contact to CIGSabsorber layer 120, ease of bonding to tabs for external connectivity,ease of scribing or other removal, good thermo-mechanical stability, andchemical resistance during subsequent processing, among properties. Backcontact layer 110 may be formed by any number of depositiontechnologies, including physical vapor deposition (PVD) or “sputtering,”evaporation, chemical vapor deposition (CVD), atomic layer deposition(ALD), plating, etc.

CIGS absorber layer 120 is a p-type absorber layer having a thickness ofbetween about 1.0 micron and about 4.0 microns and includes acopper-indium-gallium-selenide material formed according to embodimentsof the invention. Specifically, CIGS absorber layer 120 has adouble-graded bandgap profile 125, which is illustrated schematically inbandgap profile diagram 150 shown in FIG. 1. For clarity, bandgapprofile diagram 150 is disposed adjacent to and aligned with CIGSabsorber layer 120 to better illustrate the change in the bandgap value126 of CIGS absorber layer 120 with respect to a light-receiving surface121 and a back contact surface 122 of CIGS absorber layer 120. Bandgapvalue 126 represents the energy difference between the top of thevalence band and the bottom of the conduction band in CIGS absorberlayer 120. As shown in bandgap profile diagram 150, bandgap profile 125is “double-graded,” i.e., the bandgap value 126 increases towardlight-receiving surface 121 and also toward back contact surface 122,with a bandgap minimum located in a center region of the CIGS absorberlayer 120. Because bandgap value 126 increases at light-receivingsurface 121, the generation of carriers near light-receiving surface 121is discouraged, thereby advantageously reducing recombination. Becausebandgap value 126 increases at back contact surface 122, a back surfacefield is created, which reduces recombination at back contact surface122 and enhances carrier collection. Bandgap profile 125 isdouble-graded since the concentration of gallium in CIGS absorber layer120 is also double-graded, with an increased concentration of gallium atlight-receiving surface 121 and also at back contact surface 122.

According to embodiments of the invention, CIGS absorber layer 120 isformed on back contact layer 110 in a two-step process that does notinclude sulfur incorporation. It is noted that embodiments of theinvention produce a double-graded concentration of gallium in CIGSabsorber layer 120 so that CIGS absorber layer 120 has a double-gradedbandgap profile 125, as illustrated in bandgap profile diagram 150.First, a precursor film that includes copper, indium, and gallium isdeposited on back contact layer 110, with co-sputtering, evaporation,electroplating, solution-based synthesis, or other metal depositionprocesses known in the art. For example, a co-sputtering process may beperformed using binary copper-gallium and indium sputter targets. Thecopper-indium-gallium precursor film may comprise multiple layers or asingle layer, and may be a dense or porous film. Subsequent todeposition of the copper-indium-gallium precursor film, one of severalpossible embodiments of the invention is used to form CIGS absorberlayer 120 with a double-graded concentration of gallium, so that CIGSabsorber layer 120 has double-graded bandgap profile 125.

In one embodiment, a Ga_(x)Se_(y) layer is formed on thecopper-indium-gallium precursor film using trimethyl gallium (TMGa) gasand a selenium (Se) containing gas in an initial, low-temperatureselenization process, in which the selenium-containing gas used in theselenization process does not react with the copper-indium-galliumprecursor film. A subsequent high-temperature selenization process formsthe typical gallium-rich region at back contact surface 122 of CIGSabsorber layer 120 while simultaneously converting the depositedGa_(x)Se_(y) layer to a gallium-rich region at light-receiving surface121 of CIGS absorber layer 120. The embodiment is described in greaterdetail below in conjunction with FIG. 2 and FIGS. 3A-3C.

In another embodiment, the copper-indium-gallium precursor film firstundergoes a conventional selenization process to form a CIGS absorberlayer that has the typical gallium-rich region at back contact surface122 and therefore has a single-graded bandgap profile. Then, using TMGaand a selenium-containing gas, a Ga_(x)Se_(y) layer is formed onlight-receiving surface 121 of the single-graded CIGS absorber layer,and a subsequent anneal process converts the deposited Ga_(x)Se_(y) to agallium-rich region at the front surface of the absorber. The embodimentis described in greater detail below in conjunction with FIG. 4 andFIGS. 5A-5D.

In yet another embodiment, the copper-indium-gallium precursor filmfirst undergoes a conventional selenization process to form a CIGSabsorber layer that has the typical gallium-rich region at back contactsurface 122 and a single-graded bandgap profile. Then, thermal pyrolysisof TMGa is used to deposit a gallium layer on light-receiving surface121 of the single-graded CIGS absorber layer, and a subsequentselenization process forms a Ga_(x)Se_(y) layer from the depositedgallium. Depending on the temperature of the selenization process, ananneal process may be used in a final step to convert the Ga_(x)Se_(y)layer to a gallium-rich region at light-receiving surface 121 of CIGSabsorber layer 120. The embodiment is described in greater detail belowin conjunction with FIGS. 6 and 7A-7E.

Buffer layer 130 of TFPV device 100 is an n-type buffer layer depositedon CIGS absorber layer 120. In one embodiment, buffer layer 130comprises a cadmium sulfide (CdS) layer that has a thickness betweenabout 30 nm and about 100 nm. Other n-type buffer layer materialssuitable for use in buffer layer 130 include ZnS, In₂S₃, In₂(S,Se)₃,CdZnS, ZnO, Zn(O,S), (Zn,Mg)O, etc. Buffer layer 130 may be depositedusing chemical bath deposition (CBD), chemical surface deposition (CSD),PVD, printing, plating, ALD, ion-layer-gas-reaction (ILGAR), orevaporation.

TCO stack 140 serves as part of the front contact structure of TFPVdevice 100 and is formed from transparent conductive metal oxidematerials. TCO stack 140 collects charge across the face of TFPV device100 and conducts the charge to tabs used to connect TFPV device 100 toexternal loads. TCO stack 140 includes a low resistivity top TCO layer142 and an optional intrinsic zinc oxide (iZnO) layer 141. Optionalintrinsic zinc oxide layer 141 is a high resistivity material that hasbeen found to reduce sensitivity of TFPV device to lateralnon-uniformities caused by differences in composition or defectconcentration in the absorber and/or buffer layers. Optional intrinsiczinc oxide layer 141 is formed on CIGS absorber layer 120 and isgenerally between 40 to 60 nm in thickness, but in some embodiments isup to about 150 nm in thickness. Optional intrinsic zinc oxide layer 141is typically formed using deposition processes well-known in the art,including reactive PVD, CVD, plating, or printing. Low resistivity topTCO layer 142 is formed on optional intrinsic zinc oxide layer 141, andtypically has a thickness between about 100 nm and 1 micron. Suitablematerials for low resistivity top TCO layer 142 include aluminum-dopedzinc oxide (Al:ZnO), indium tin oxide (InSnO or ITO), indium zinc oxide(InZnO), boron-doped zinc oxide (B:ZnO), gallium-doped zinc oxide(Ga:ZnO), fluorine-doped zinc-oxde (F:ZnO), fluorine-doped tin oxide(F:SnO₂), etc. Suitable processes for forming low resistivity top TCOlayer 142 include reactive PVD, CVD, printing or wet-coating fromnano-wires or carbon nanotubes, and the like.

FIG. 2 sets forth a flowchart of method steps in a process sequence 200for forming CIGS absorber layer 120, according to embodiments of theinvention. FIGS. 3A-3C sequentially illustrate cross-sectional views ofTFPV device 100 during the execution of process sequence 200, accordingto embodiments of the invention. Although the method steps are describedin conjunction with TFPV device 100, persons skilled in the art willunderstand that formation of other TFPV devices using process sequence200 is within the scope of the invention. Prior to the first step ofmethod 200, back contact layer 110 is deposited on substrate 105, and acopper-indium-gallium precursor film 301 is formed on back contact layer110 (shown in FIG. 3A). As shown in bandgap profile diagram 350A in FIG.3A, the bandgap profile 325A of copper-indium-gallium precursor film 301is substantially constant from back contact surface 122 to a frontsurface 321 of copper-indium-gallium precursor film 301. In an exemplaryembodiment, copper-indium-gallium precursor film 301 has a thickness of200 nm to 1000 nm and a composition range of copper to indium andgallium of 0.7 to 0.95 and gallium to gallium and indium of 0.1 to 0.4.

As shown in FIG. 2, method 200 begins at step 201, in which the reactionof TMGa vapor with a selenium-containing gas is used to form aGa_(x)Se_(y) layer 302 on copper-indium-gallium precursor film 301,(illustrated in FIG. 3B). In addition, the reaction takes place betweenTMGa vapor and the selenium-containing gas is performed at a relativelylow temperature, so that the selenium-containing gas used in theselenization process does not react with copper-indium-gallium precursorfilm 301. In such a selenization process, the temperature of substrate105 is maintained below the threshold reaction temperature for theselenization of copper-indium-gallium precursor film 301, so thatGa_(x)Se_(y) layer 302 is formed on copper-indium-gallium precursor film301 as shown in FIG. 3B. Bandgap profile diagram 350B in FIG. 3Billustrates that bandgap profile 325B of copper-indium-gallium precursorfilm 301 remains substantially constant, since little or no galliumdiffuses into copper-indium-gallium precursor film 301 during step 201.However, the high concentration of gallium in Ga_(x)Se_(y) layer 302results in a higher bandgap at light-receiving surface 121.

Various selenium-containing gases may be used in step 201 to formGa_(x)Se_(y) layer 302, including hydrogen selenide (H₂Se), seleniumvapor, and/or diethylselenide. In one embodiment, step 201 takes placein either a batch furnace or an in-line furnace at a depositiontemperature between about 20° C. and about 350° C., and hydrogenselenide is used as the selenium-containing gas. In such an embodiment,Ga_(x)Se_(y) layer 302 is deposited with a thickness of about 10 nm to100 nm using the reaction described in Equation 1:3(CH₃)₃Ga_((g))+4H₂Se_((g))½H_(2(g))→Ga₂Se_(3(s))+GaSe_((s))+9CH_(4(g))  (1)

In step 202, selenization of copper-indium-gallium precursor film 301and Ga_(x)Se_(y) layer 302 is performed by reaction with aselenium-containing gas that comprises hydrogen selenide, seleniumvapor, diethylselenide, and/or a combination thereof. The selenizationprocess of step 202 forms CIGS absorber layer 120 fromcopper-indium-gallium precursor film 301 and Ga_(x)Se_(y) layer 302, asshown in FIG. 3C. The reaction with the selenium-containing gas takesplace at an elevated temperature (e.g., between about 400° C. and 550°C.) so that gallium present in copper-indium-gallium precursor film 301reacts with the selenium-containing gas to form a CIGS absorber layerthat is gallium-rich at back contact surface 122. However, at suchreaction temperatures, gallium contained in Ga_(x)Se_(y) layer 302 haslimited mobility due to the strong Ga—Se bond, and remains concentratednear light-receiving surface 121 of CIGS absorber layer 120.Consequently, upon completion of step 202, CIGS absorber layer 120 has adouble-graded bandgap profile 325C, as illustrated in bandgap profilediagram 350C of FIG. 3C.

In one embodiment, the selenium-containing gas comprises hydrogenselenide and the reaction temperature is between about 400° C. and 550°C. In an another embodiment, the selenium-containing gas comprisesselenium vapor and the reaction temperature is between about 400° C. and600° C. It is noted that the processes described for step 202 may beperformed in the same batch furnace or in-line furnace that performs theprocesses of step 201. Consequently, implementation of method 200 issubstantially more economical and less complex than processes in whichmultiple processing chambers are required for the formation of CIGSabsorber layer 120.

In optional step 203, double-graded bandgap profile 325C illustrated inFIG. 3C is tuned or optimized in a final anneal process. The annealprocess of step 203 can adjust gallium distribution in CIGS absorberlayer 120, thereby altering the double-graded bandgap profile 325C. Forexample, in one embodiment, the anneal process of step 203 adjustsdouble-graded bandgap profile 325C to a double-graded bandgap profile325D. In some embodiments, the anneal process of step 203 is performedat a temperature greater than or equal to 500° C. It is noted that insome embodiments, depending on the reaction temperature and duration ofthe selenization process of step 202, step 203 may not be necessary.Specifically, in some embodiments, double-graded bandgap profile 325Cmay be adjusted to double-graded bandgap profile 325D during theselenization process of step 202, i.e., when the selenization process ofstep 202 takes place at a sufficiently high temperature and for asufficiently long duration.

FIG. 4 sets forth a flowchart of method steps in a process sequence 400for forming CIGS absorber layer 120, according to embodiments of theinvention. FIGS. 5A-5D sequentially illustrate cross-sectional views ofTFPV device 100 during the execution of process sequence 400, accordingto embodiments of the invention. Although the method steps are describedin conjunction with TFPV device 100, persons skilled in the art willunderstand that formation of other TFPV devices using process sequence400 is within the scope of the invention. Prior to the first step ofmethod 400, back contact layer 110 and copper-indium-gallium precursorfilm 301 are deposited on substrate 105 as described above in method200. Consequently, in bandgap profile diagram 550A in FIG. 5A, thebandgap profile 525A of copper-indium-gallium precursor film 301 issubstantially constant over the thickness of copper-indium-galliumprecursor film 301.

As shown in FIG. 4, method 400 begins at step 401, in which aselenization process known in the art is performed oncopper-indium-gallium precursor film 301 to form a CIGS layer 501, asshown in FIG. 5B. Due to gallium's slower reaction kinetics withselenium compared to indium, gallium accumulates towards back contactsurface 122 of CIGS layer 501 during the selenization process, so thatCIGS layer 501 has a single-graded bandgap profile that increases fromfront surface 521 of CIGS layer 501 to back contact surface 122. Bandgapprofile diagram 550B in FIG. 5B illustrates the single-graded bandgapprofile 525B of CIGS layer 501. Selenization processes suitable for usein step 401 may be performed in a batch furnace or in-line furnace, aretypically carried out in a temperature range of approximately 400° C. to500° C., and generally use hydrogen selenide and/or selenium vapor.

In step 402, the reaction of TMGa vapor with a selenium-containing gasis used to form a Ga_(x)Se_(y) layer 502 on CIGS layer 501, illustratedin FIG. 5C. The process of forming Ga_(x)Se_(y) layer 502 on CIGS layer501 is similar to the process of forming Ga_(x)Se_(y) layer 302 oncopper-indium-gallium precursor film 301 in step 201 of method 200, butdiffers in one respect. Specifically, the reaction described in Equation1 can be performed in step 402 at a higher temperature than in step 201of method 200, e.g., up to about 550° C., since CIGS layer 501 isalready selenized in step 401. As illustrated by bandgap profile diagram550C, bandgap profile 525C of CIGS layer 501 has a single grade thatincreases toward back contact surface 122, while the high concentrationof gallium in Ga_(x)Se_(y) layer 502 results in a correspondingly higherbandgap at light-receiving surface 121. It is noted that the processesdescribed for step 402 may be performed in the same batch furnace orin-line furnace that performs the processes of step 401.

In step 403, an anneal process is performed on CIGS layer 501 andGa_(x)Se_(y) layer 502 to form CIGS absorber layer 120, as illustratedin FIG. 5D. The anneal process of step 403 converts Ga_(x)Se_(y) layer502 into a gallium-rich CIGS region at light-receiving surface 121, sothat the bandgap of CIGS absorber layer 120 increases at light-receivingsurface 121. Thus, in step 403, CIGS absorber layer 120 is formed with adouble-graded bandgap profile, as illustrated by bandgap profile 525D inbandgap profile diagram 550D. The duration and temperature at which theanneal process of step 403 takes place may be selected to adjust bandgapprofile 525D as desired. In one embodiment, the anneal process of step403 is performed at a temperature between about 500° C. and about 600°C. It is noted that the anneal process of step 403 may be performed inthe same batch furnace or in-line furnace that performs the processes ofsteps 401 and 402.

FIG. 6 sets forth a flowchart of method steps in a process sequence 600for forming CIGS absorber layer 120, according to embodiments of theinvention. FIGS. 7A-7D sequentially illustrate cross-sectional views ofTFPV device 100 during the execution of process sequence 600, accordingto embodiments of the invention. Although the method steps are describedin conjunction with TFPV device 100, persons skilled in the art willunderstand that formation of other TFPV devices using process sequence600 is within the scope of the invention. Prior to the first step ofmethod 600, back contact layer 110 and copper-indium-gallium precursorfilm 301 are deposited on substrate 105 as described above in method200. Consequently, in bandgap profile diagram 750A (FIG. 7A), thebandgap profile 725A of copper-indium-gallium precursor film 301 issubstantially constant over the thickness of copper-indium-galliumprecursor film 301.

As shown in FIG. 6, method 600 begins at step 601, in which aselenization process known in the art is performed oncopper-indium-gallium precursor film 301 to form a CIGS layer 701, asshown in FIG. 7B. Step 601 is substantially similar to step 401 inmethod 400, so that CIGS layer 701 has a single-graded bandgap profilethat increases from front surface 521 of CIGS layer 701 to back contactsurface 122. Bandgap profile diagram 750B in FIG. 7B illustrates thesingle-graded bandgap profile 725B of CIGS layer 701.

In step 602, a gallium layer 702 is formed on CIGS layer 701,illustrated in FIG. 7C. Gallium layer 702 is formed by exposing CIGSlayer 701 to TMGa vapor at a temperature between about 400° C. and 550°C. Thermal pyrolysis of the TMGa vapor results in the deposition ofgallium layer 702. It is noted that the processes described for step 602may be performed in the same batch furnace or in-line furnace thatperforms the processes of step 601. Bandgap profile diagram 750C (FIG.7C) illustrates the single-graded bandgap profile 725C of CIGS layer 701and the high bandgap value at light-receiving surface 121 associatedwith gallium layer 702.

It is noted that the formation of gallium layer 702 in step 602 resultsfrom the thermal decomposition of TMGa on the exposed surface ofcopper-indium-gallium precursor film 301. This is in contrast to theformation of Ga_(x)Se_(y) layer 502 on copper-indium-gallium precursorfilm 301, in step 402 of method 400, which is a gas-phase reaction thatcan potentially create unwanted particles the reaction chamber.

In step 603, a selenization process is performed with aselenium-containing gas. Selenization processes suitable for use in step603 may be performed in a batch furnace or in-line furnace, aretypically carried out in a temperature range of approximately 400° C. to550° C., and generally use hydrogen selenide and/or selenium vapor. Theselenization process of step 603 converts gallium layer 702 to aGa_(x)Se_(y) layer 703, illustrated in FIG. 7D, and produces the desireddouble-graded bandgap profile, as illustrated by bandgap profile 725D inbandgap profile diagram 750D.

In optional step 604, an anneal process is performed on CIGS absorberlayer 120. The anneal process of step 604 may be used to furtheroptimize or adjust the bandgap profile of CIGS absorber layer 120. Insome embodiments, the anneal process in step 604 is performed at atemperature between about 500° C. and 600° C.

It is noted that the processes described for steps 601-604 may all beperformed in the same batch furnace or in-line furnace. Consequently,implementation of method 700 is substantially more economical and lesscomplex than processes in which multiple processing chambers arerequired for the formation of CIGS absorber layer 120. It is furthernoted that embodiments of the invention may be performed using anytechnically feasible deposition techniques known in the art. Forexample, TFPV device 100 may be formed using single substrate processingequipment, multiple substrate batch-processing equipment, in-lineprocessing, single chamber processing, roll-to-roll processing, and thelike. In-line processing may include continuous processing of substrateswhile moving through an in-line furnace, or the performance of differentprocesses on each substrate in multiple discrete reaction chambers. Suchchambers may be isolated mechanically, by gas curtains, etc.

In sum, embodiments of the invention set forth methods for forming aCIGS absorber layer in a thin film photovoltaic device, where the CIGSabsorber layer has a double-graded bandgap and a double-graded galliumconcentration. One advantage of the invention is that the formation of aCIGS absorber layer with a double-graded bandgap can, according tovarious embodiments of the invention, be performed sequentially in asingle reaction chamber or furnace. Thus, a CIGS absorber layer havingsuperior conversion efficiency can be formed in an economical fashion.

While the foregoing is directed to embodiments of the present invention,other and further embodiments of the invention may be devised withoutdeparting from the basic scope thereof, and the scope thereof isdetermined by the claims that follow.

I claim:
 1. A method of forming a thin-film solar cell absorber layer,the method comprising: forming a first layer above the substrate,wherein the first layer comprises copper, indium, gallium, and selenium;prior to forming a second layer above the first layer, heating thesubstrate to form a gallium-rich region within the first layer, whereinthe step of heating the substrate prior to forming the second layercomprises a selenization process; forming a second layer above the firstlayer, wherein the second layer is formed using trimethyl gallium (TMGa)and a selenium-containing gas; and heating the first layer and thesecond layer.
 2. The method of claim 1, wherein exposing the first layerto TMGa and the selenium-containing gas and the heating the first layerand the second layer are performed in a same reaction chamber.
 3. Themethod of claim 1, wherein the heating the first layer and the secondlayer comprises heating to a temperature between 500° C. and 600° C. 4.The method of claim 1, wherein the selenium-containing gas compriseshydrogen selenide (H₂Se).
 5. The method of claim 1, wherein aconcentration within the absorber layer of copper to indium and galliumis between 0.7 and 0.95.
 6. The method of claim 1, wherein aconcentration within the absorber layer of gallium to indium and galliumis between 0.1 and 0.4.
 7. The method of claim 1, wherein a thickness ofthe first layer is between 200nm and 1000nm.